1. Field of the Invention
The present invention generally relates to deposition of a metal film onto a substrate. More particularly, the present invention relates to a flow diffuser to be used within an electrochemical plating (ECP) that electroplates a metal film on a seed layer on a substrate.
2. Description of the Related Art
As circuit densities increase, the widths of features such as vias and electric, contact elements, as well as the width of the dielectric materials between the features, have decreased over recent years. Electroplating, previously limited in integrated circuit design to the fabrication of lines on circuit boards, is now used to deposit metal films, such as copper, on substrates to form features. One feature filling embodiment that utilizes electroplating requires initially depositing a diffusion barrier layer on the substrate by a process such a physical vapor deposition (PVD) or chemical vapor deposition (CVD). A seed layer is deposited on the diffusion barrier layer by a process such as PVD or CVD to define a plating surface on the substrate. Metal ions are then deposited by electroplating on the substrate seed layer to form a metal film. Finally, the deposited metal film can be planarized by process, e.g., chemical mechanical polishing (CMP), to define a conductive interconnect feature. Electroplating is performed by establishing a voltage/current level between the seed layer on the substrate and a separate anode to deposit metal ions on the layer to form the deposited metal film.
Electrolyte solution is injected into in an electrolyte cell used for electroplating from an inlet disposed below the anode, and the electrolyte solution follows a generally upward path. The fluid flow pattern of the electrolyte solution flowing around the anode plate to the upper surface of the anode plate is typically non-linear considering the shape and contour of the anode and the irregular fluid flow path around the anode. Providing as smooth of a fluid flow of electrolyte solution to the upper surface of the anode enhances the generation of metal ions in the electrolyte solution. Turbulence in the electrolyte solution flow adjacent the upper surface of the anode plate causes in eddies to form near the upper surface of the anode plate. The formation of such eddies obstructs the chemical reaction between the electrolyte solution and the anode. Electrolyte solution having a laminar flow interacts more uniformly and predictably with the anode than electrolyte solution having a turbulent flow. Since chemical reaction releases metal ions from the anode into the electrolyte solution, providing a turbulent flow adjacent the upper anode surface limits the chemical reaction between the electrolyte solution and the anode, and thereby reduces the quantity of metal ions released into reducing the quantity of metal ions released into the electrolyte solution limits the plating effectiveness. It is therefore desired to enhance the laminar flow characteristics forming adjacent the upper surface of the anode to improve the uniformity of metal ion generation by the anode into the electrolyte solution.
The fluid flow of the electrolyte solution from the anode to the seed layer on the substrate is generally directed perpendicularly upward towards the seed layer. One technique to provide uniformity of flow across the width of the electrolyte cell involves extending a diffuser across the width of the electrolyte cell. The diffuser is typically formed as a sheet or layer of permeable plastic. The diffuser is configured to permit the electrolyte solution containing the metal ions to pass through. Fluid pressure applied to the permeable plastic of the diffuser flexes or deforms the diffusers due to the flexibility of the diffuser. These prior art diffusers therefore bow or deform to assume an upwardly facing convex shape when electrolyte solution flows upwardly through the diffuser from below. The upward-directed convex configuration of the diffuser results in those portions of the diffuser being positioned about the center of the diffuser being located closer to the nearest location on the seed layer than those portions of the diffuser located about the periphery. The electric resistance of the electrolyte solution varies as a function of distance through the electrolyte solution. The electric current flow from the flow diffuser via the electrolyte solution to the seed layer is therefore increased at the nearest location on the seed layer since the electric resistance of the electrolyte solution diminishes as distance decreases. As such, the electric current density applied to the seed layer at the center of the substrate may be enhanced relative to the electric current density of those seed layer portions adjacent the periphery of the substrate.
Therefore, there remains a need for an electrochemical plating system having a diffuser that enhances the uniformity of electric current density applied across the face of the seed layer. In one aspect, the diffuser would enhance the fluid flow to the upper surface of the anode to limit the turbulence, and thereby enhance the chemical reaction of the anode relative to the electrolyte solution.